1. Field of the Invention
The present invention generally relates to magnetic recording media, magnetic storage apparatuses and recording methods, and more particularly to a magnetic recording medium which is suited for high-density recording, a magnetic storage apparatus which uses such a magnetic recording medium, and a recording method for recording information on such a magnetic recording medium.
2. Description of the Related Art
Recently, the recording densities of magnetic recording media have increased rapidly, even at a rate reaching 100% per year. However, in the popularly employed longitudinal (or in-plane) recording system, it is expected that a limit of the longitudinal recording density will be on the order of 100 Gb/in2, because of problems associated with thermal stability of the magnetic recording medium. In order to reduce the medium noise in the high-density recording region, the size of crystal grain forming the magnetization unit is reduced, so as to reduce the zigzag of the boundary between the magnetization units, that is, the magnetization transition region. However, when the size of the crystal grain is reduced, the volume forming the magnetization unit decreases, to thereby cause the magnetization to decrease due to thermal instability. Accordingly, in order to achieve a high recording density exceeding 100 Gb/in2, it is necessary to simultaneously reduce the medium noise and improve the thermal stability.
Magnetic recording media which simultaneously reduce the medium noise and improve the thermal stability have been proposed in Japanese Laid-Open Patent Applications No. 2001-056921 and No. 2001-056924, for example. FIG. 1 is a cross sectional view showing a part of a proposed magnetic recording medium 100. The proposed magnetic recording medium 100 shown in FIG. 1 includes an exchange layer structure provided on a substrate 105, and a magnetic layer 102 provided on the exchange layer structure. The exchange layer structure is made up of a ferromagnetic layer 101 provided on the substrate 105, and a nonmagnetic coupling layer 103 provided on the ferromagnetic layer 101. The ferromagnetic layer 101 and the magnetic layer 102 are exchange-coupled anti-ferromagnetically via the nonmagnetic coupling layer 103. The effective crystal grain volume becomes the sum of crystal grain volumes of the ferromagnetic layer 101 and the magnetic layer 102 which are exchange-coupled. Consequently, the thermal stability is greatly improved, and the medium noise can be reduced because the crystal grain size can further be reduced. By using the proposed magnetic recording medium 100, the thermal stability of the recorded (written) bits improve, and the medium noise is reduced, thereby enabling a highly reliable high-density recording.
In the proposed magnetic recording medium 100, the reproduced output is approximately proportional to a difference between the remanent magnetizations of the magnetic layer 102 and the ferromagnetic layer 101, because the magnetization directions of the magnetic layer 102 and the ferromagnetic layer 101 are mutually antiparallel. Hence, in order to obtain a reproduced output comparable to that obtained by the conventional magnetic recording medium having the magnetic layer with the single-layer structure, the magnetic layer 102 closer to a recording and/or reproducing magnetic head is set thicker than the ferromagnetic layer 101 which is further away from the magnetic head, and also thicker than the conventional magnetic layer having the single-layer structure, if materials having the same composition are used for the magnetic layer 102 and the ferromagnetic layer 101. However, when the proposed magnetic recording medium 100 has the magnetic layer 102 with such a thickness, there is a possibility of deteriorating the write performances, such as the overwrite performance and the Non-Linear-Transition-Shift (NLTS) performance, due to the increased thickness of the magnetic layer 102.
On the other hand, when a recording magnetic field is applied to the proposed magnetic recording medium 100 from the magnetic head at the time of the recording, the magnetization directions of the magnetic layer 102 and the ferromagnetic layer 101 align in the direction of the recording magnetic field and become mutually parallel. Thereafter, when the magnetic head moves and the recording magnetic field weakens, the magnetization direction of the ferromagnetic layer 101 switches in response to an exchange field of the magnetic layer 102 and the magnetization directions of the ferromagnetic layer 101 and the magnetic layer 102 become mutually antiparallel. However, in a vicinity of a magnetic pole of the magnetic head at a trailing edge along the moving direction of the magnetic head, the behaviors of the magnetic layer 102 and the ferromagnetic layer 101, such as the switching of the magnetization directions, immediately after switching the direction of the recording magnetic field, become complex due to the exchange field and the demagnetization field of each of the magnetic layer 102 and the ferromagnetic layer 101. With respect to the magnetic layer 102, the position, inclination and the like of the magnetization transition region may change and the NLTS performance may deteriorate, particularly due to the magnetic characteristics and the like of the ferromagnetic layer 101.